Fidelity Institutional Advisors: How to Open a Solo 401(k) in 2026 and Still Fund for 2025

If you’re a self-employed professional—or a financial advisor working with self-employed clients—you may be surprised to learn that it’s still possible to open a Solo 401(k) in 2026 and make contributions that count for tax year 2025.

Watch: Fidelity Advisors can opne a solo 401k in 2026 for 2025 using My Solo 401k Financial as the Plan Provider

This strategy is especially powerful when combined with Fidelity Institutional Advisors, where the advisor manages investments while an independent Solo 401(k) plan provider handles plan documents and IRS compliance.

This article breaks down how the strategy works, who qualifies, and why Fidelity Institutional Advisors frequently partner with MySolo401k Financial for advanced Solo 401(k) structures.

Why a Solo 401(k) Is the Most Powerful Plan for the Self-Employed

A Solo 401(k) is designed for owner-only businesses with no non-owner, full-time W-2 employees working 1,000+ hours per year. Independent contractors do not count as employees for eligibility purposes.

Eligible business owners can include:

Sole proprietors and single-member LLCs
S-corporations
Partnerships (including husband-and-wife businesses)

When structured properly, a Solo 401(k) allows:

Employee contributions (pre-tax or Roth, when timely adopted)
Employer profit-sharing contributions
Voluntary after-tax contributions for Mega Backdoor Roth strategies
Roth conversions
Participant loans
Self-directed investing, including alternatives

Why Fidelity Institutional Advisors Use an Independent Solo 401(k) Plan

Fidelity Institutional provides custody and investment access, but it does not supply a Solo 401(k) plan document that supports advanced features like:

Voluntary after-tax contributions
Mega Backdoor Roth conversions
Multiple sub-accounts (pre-tax, Roth, after-tax)
Custom compliance support

That’s where an independent plan document provider comes in.

With this structure:

MySolo401k Financial provides the IRS-approved Solo 401(k) plan documents and compliance support
Fidelity Institutional Advisors custody assets and manage investments
The advisor and client maintain full control of the assets
The plan—not Fidelity—dictates what strategies are allowed

You Can Open a Solo 401(k) in 2026 and Still Fund 2025

Thanks to IRS and SECURE Act 2.0 rules, Solo 401(k) plans can be adopted after year-end and still accept certain prior-year contributions.

Contribution Deadlines by Entity Type

S-Corporation or Partnership

Plan opened in 2026
Can still make 2025 employer profit-sharing contributions
Can still make 2025 voluntary after-tax contributions
Deadline: business tax filing date (March 15, 2026 or Sept 15, 2026 with extension)
❌ Employee deferrals for 2025 are no longer allowed

Sole Proprietor / Single-Member LLC

Plan opened by April 15, 2026
Can make employee, employer, and after-tax contributions for 2025
With extension, employer + after-tax contributions allowed until October 15, 2026

This is one of the most misunderstood—but powerful—planning opportunities for self-employed individuals.

Mega Backdoor Roth with Fidelity Institutional

A properly designed Solo 401(k) can support the Mega Backdoor Roth strategy, even though Fidelity’s prototype Solo 401(k) does not.

Here’s how it works:

Open a Solo 401(k) with a plan document that allows voluntary after-tax contributions
Open separate Fidelity Institutional brokerage accounts for:
- Pre-tax funds
- Roth funds
- Voluntary after-tax funds
Make voluntary after-tax contributions (up to the annual limit, based on income)
Convert those funds to:
- Roth Solo 401(k), or
- Roth IRA

For 2025, the overall Solo 401(k) limit is $70,000 (not including catch-up). That entire amount can potentially be routed through the Mega Backdoor Roth if income supports it.

Married Business Owners: Double the Strategy

If both spouses work in the same owner-only business:

One Solo 401(k) plan
Two participants
Separate sub-accounts for each spouse

That means up to $70,000 per spouse for 2025—$140,000 total—potentially converted to Roth using Mega Backdoor strategies.

Bank Accounts vs. Brokerage Accounts

A Solo 401(k) can have multiple holding accounts, including:

Brokerage accounts
Bank accounts

Most clients use only Fidelity brokerage accounts, which can hold:

Stocks, ETFs, mutual funds
CDs and money market funds
Alternative investments (when properly structured)

Bank accounts are typically only needed for niche use cases like:

Real estate auctions (cashier’s checks)
Tax lien investing

Funds can move between accounts within the same plan without triggering taxes or reporting.

Compliance, Reporting, and Support

With a properly structured Solo 401(k):

The plan has its own EIN (separate from the business)
Roth conversions are reported on Form 1099-R
Form 5500-EZ is required once plan assets exceed $250,000
Ongoing IRS-required plan updates are handled

The advisor manages investments.
The plan provider manages compliance.
The client keeps control.

Final Thoughts

Opening a Solo 401(k) in 2026 does not mean you missed out on 2025 tax benefits. When structured correctly—especially with Fidelity Institutional Advisors—you can still: